This publication is a joint effort of the Maryland Sea Grant College and the Alliance for the Chesapeake Bay, supported by the Chesapeake Bay Program's Toxics Subcommittee

A Toxic Primer

Even the terms we use to speak about "toxics" can be confusing – and confusion often leads to miscommunication and misunderstanding. Consider the important but often poorly understood differences among the following terms.

Toxics. Although "toxic" appears in the dictionary as an adjective, it does not appear as a noun. This is important, because while an adjective describes the character or attribute of, say, a chemical compound, the term as a noun suggests that a compound is by definition toxic. For example, consider that oxygen is deadly for a number of microbial life forms (anaerobes, which once dominated the earth), but no one would describe oxygen as a toxic. The same point could be made about many chemicals in and around the Bay, and one might say that toxics are simply chemicals in the wrong place. It is therefore important to remember that calling a compound a toxic may be misleading, and that toxicity will depend on several factors:

• the concentration (dose) of a specific contaminant in a form determined to be "biologically reactive" in a particular environment
  
  
• how frequently an organism is exposed (exposure frequency)
  
  
• how long an organism is exposed (exposure duration)

Organisms will tend to vary in terms of their sensitivity to these factors, and their response may well depend on environmental conditions, such as temperature and salinity. While some potential contaminants are beneficial in small amounts – zinc is an example – others can be harmful even at very low concentrations, and these are thought of as being particularly toxic. EPA's action level for dioxin (TCDD), for example, is one part per billion.

Toxin. Although some may use the word toxin to describe toxic compounds in the open environment, the term more accurately refers to biological compounds found within an organism. Some marine organisms produce toxins, as defense mechanisms, for example.

Toxicant. This is the noun form derived from "toxic." It describes a compound or chemical that has been determined to be harmful to living organisms at certain known doses.

Contaminant. A contaminant is a substance or compound that has the potential to become toxic. Contaminants are often thought of as chemicals or compounds not found naturally in an ecosystem and which have the potential to cause harm to organisms or populations of organisms. Wastes from a variety of sources (both point and nonpoint) that cause deleterious effects in plants and animals are considered to be contaminants. In general, the words toxic and contaminant are used interchangeably, though as noted above, the word toxic is more properly an adjective, and the word contaminant a noun.

Chemical. The word chemical can be used to describe any number of compounds, though it is often assumed (at times incorrectly) that these chemical compounds are man-made and are harmful. The terms chemical and contaminant are often confused. Some chemicals are very toxic, of course, while others are very beneficial. Most medicines, for example, could be described as chemicals.

Compound. A compound is simply a combination of two or more elements (for a list of basic elements, check the periodic table in an encyclopedia or similar information source). Some compounds, such as H₂O (water), are life sustaining, while others may prove toxic at certain levels of dose and exposure, e.g., H₂ S₄ (sulfuric acid).

Pollutant. The word pollutant is not particularly scientific, and actually had a more moral sense in its first usage (e.g., "a polluted [tainted] soul"). A pollutant is now deemed to be any chemical compound that degrades the environment – by killing fish, for example, or causing disease. As with toxic or contaminant the term assumes that we already know that the compound is harmful.

Given the complexity of terms, and the limits of our understanding of biology and chemistry, some have questioned whether the goal of a "toxics-free Bay" makes any sense. But if one understands that the word "toxics" does not stand for all chemicals but only those we know to be harmful at realistic and predictable doses and that "free" represents an objective toward which we are heading, then the term serves us as a goal. The main point here is that no one wants to put contaminants into the Chesapeake Bay. The great challenge before us is to continue to improve both our understanding of what "toxic" means and our techniques for preventing unwanted chemicals and compounds from entering the Bay or its watershed.

Credits

Writers: Jack Greer, Merrill Leffler, James Riordon Designer: Sander Rodgers Contaminants and the Chesapeake Bay is a joint effort of the Maryland Sea Grant College and the Alliance for the Chesapeake Bay, supported by the Chesapeake Bay Program's Toxics Subcommittee.